According to a first aspect, the invention provides a watch middle cum plate made of brass of the kind which comprises top and bottom faces, at least one of them being formed with recesses for receiving components of a watch movement and side faces, and which is provided with a coating made of a material that is chemically more inert than brass.
Watches fitted with middles cum plates of the above kind have been described in many published patent specifications, e.g. Swiss patent specifications Nos. 296, 6502 and 250700, and French patent specification No. 2085292. So far, however, the number of such middles cum plates actually to have been produced has at best been extremely small.
As is known, brass is very often used in watch-making, in particular for making watch plates and watch cases. One advantage of this material is that it can readily be machined. On the other hand it is very subject to oxidation.
To overcome this drawback, the plates are generally coated with a layer of nickel which in turn may be clad with gold. The thickness of this layer does not usually exceed 2 .mu.m. This suffices to protect the brass while hardly affecting the dimensions of the plates.
In the manufacture of watch cases, the brass is coated with a layer of chromium, gold or any other suitable protective material. The protective layer on the cases must be much more resistant than that on the plates as it is subject to many kinds of attack, both physical and chemical. A layer of 2 .mu.m is therefore not enough. In practice, the thickness is generally of at least 5 .mu.m and may exceed 20 .mu.m. Such added thickness on the plates would considerably increase the spread in value of the dimensions and the resulting plates would be unfit for automated assembly of the components.
Thus, the production of a watch having a middle cum plate made of brass gives rise to the following dilemma: either an effective protection is provided in which case the brass needs a thick protective coating thus making assembly difficult, or a coat is provided that is sufficiently thin so as not to affect assembly, in which case the exposed surfaces of the watch are not adequately protected.
Swiss patent specifications Nos. 296 and 6502 do not discuss the problem set by this dilemma. Swiss patent specification No. 250700 does, and proposes making the plate-forming portion out of brass and the middle-forming portion out of steel, these two portions being then assembled by welding or by force-fitting. If at first this idea might seem attractive, it should be borne in mind that the considerable stresses that may be generated in so doing are likely to deform the plate-forming portion thereby making it unfit for automated assembly.
French patent specification No. 2 085 292 suggests using aluminium, copper, metal alloys and synthetic materials to produce a middle cum plate and not to apply a protective layer. None of these materials would it is believed enable sufficient accuracy to be achieved for the plate-forming portions to enable automated assembly, unless it is an alloy such as brass which, because of its propensity for oxidation, must be provided with a protective coating.